1. Field of the Invention
This invention relates to the field of signal generation in a communication system, and, more particularly, to the production of multiple output signals in a single digital exciter assembly.
2. Description of the Prior Art
It has been known in the radio broadcasting art to transmit a composite signal consisting of a frequency modulated (FM) analog signal and a digital signal, such as an in band on channel (IBOC) digital signal. The prior art includes, for example, the U.S. patents to Murphy et al. U.S. Pat. No. 5,315,583 and Papadopoulos et al. U.S. Pat. No. 6,144,705.
The prior art also includes separate generation and amplification of such signals, sometimes referred to as high-level combining. An example is illustrated in FIG. 1 herein. In this example, the output v2 of an existing first transmitter 10, illustrated as a main transmitter, is combined with the output v3 from a second transmitter 12, referred to as an auxiliary transmitter. The input to the main transmitter 10 is obtained from a first signal source 14 while the input to the auxiliary transmitter 12 is obtained from a second signal source 16. The outputs v2 and v3 of these transmitters are combined in a conventional coupler C, which typically is a 10 dB coupler. The coupler, which is sometimes known as an output coupler, provides an output v1 that is supplied to a transmitting antenna 20 for broadcasting the composite signal. The coupler C has ports 1, 2, 3, and 4 with the voltages v2 and v3 being applied to ports 2 and 3. The output at port 1 is supplied to the antenna 18. Port 4 is coupled to a reject load RL.